1. Field of the Invention
This invention relates generally to the field of multiple conversion superheterodyne receiver systems and the elimination of self-quieting spurious responses therein. More particularly, this invention relates to a ROM programmed frequency synthesized multiple conversion receiver system for improving receiver frequency stability and eliminating receiver self-quieting spurious responses via phase-locked oscillators.
2. Background of the Invention
The receiver system designer is frequently confronted with two serious problems when designing a multiple conversion superheterodyne receiver which must receive a wide range of radio frequency inputs. The first such problem is known as receiver self-quieting spurious responses while the second problem is that of receiver frequency stability. For wideband receivers the frequency stability problem is compounded by the requirement for high intermediate frequencies in such wideband receivers.
The receiver self-quieting phenomenom is caused by harmonics of the receiver's local oscillators mixing together in any of the non-linear stages of the receiver to produce a frequency which the receiver is capable of responding to as though it were an incoming or intermediate frequency signal. This receiver self-quieting phenomenom is prehaps best understood by first examining the phenomenom in a conventional dual conversion superheterodyne receiver such as the one shown in FIG. 1. In this system an RF frequency input F.sub.RF first enters input 10 of the receiver's RF stages 15. These RF stages 15 may include amplification, matching, filtering networks, etc. as required by the system. In general, substantially the same RF frequency F.sub.RF will exit an output 20 of RF stages 15 and enter an input 25 of a first mixer 30.
A first local oscillator frequency F.sub.LO1 is produced by a first local oscillator 35 having an output 40 operatively coupled to a second input 45 of first mixer 30. Oscillator 35 may be a conventional crystal controlled oscillator whose frequency is determined by a crystal 50. This oscillator crystal may be one of many such crystals which may be selectively coupled to oscillator 35 in order to provide the user with a plurality of receiver channels. Alternatively, oscillator 35 may be a frequency synthesizer which may generate any number of frequencies via frequency synthesis in order to provide the user with a plurality of receiver channels.
As is well known in the art, first mixer 30 will produce an output intermediate frequency F.sub.IF1 on output terminal 55 in accordance with the equation F.sub.RF -F.sub.LO1 =F.sub.IF1 if the system uses low side injection to the first mixer or F.sub.IF1 =F.sub.LO1 -F.sub.RF if the system utilizes high side injection at first mixer 30.
This first intermediate frequency F.sub.IF1 is applied to an input 60 of first I.F. stages 65. First I.F. stages 65 may include amplifiers and filters for processing the intermediate frequency signal F.sub.IF1 as necessary. In the preferred embodiment first I.F. stages 65 includes a narrow-band crystal filter.
An output 70 of the first I.F. stages 65 is operatively coupled to an input 75 of a second mixer 80 thereby applying F.sub.IF1 thereto. A second local oscillator 85 provides a second local oscillator frequency F.sub.LO2 at an output 90 to be applied to an input 95 of second mixer 80. Oscillator 85 is generally a fixed frequency oscillator having frequency F.sub.LO2 determined by a single oscillator crystal 100.
A second intermediate frequency appears at an output 105 of mixer 80 and has frequency designated F.sub.IF2. The second local oscillator frequency F.sub.LO1 is determined in accordance with the equation F.sub.IF2 =F.sub.IF1 -F.sub.LO2 if low side injection is utilized for the second mixer and F.sub.LO2 -F.sub.IF1 =F.sub.IF2 if high side injection is utilized for the second mixer.
This second I.F. frequency F.sub.IF2 is applied to an input 100 of second I.F. stages 115 where the signal is further processed and appears at an output 120 of the second I.F. stages. At this point the signal is further processed by other circuitry as deemed necessary in accordance with the systems specifications and requirements. Most frequently output 120 will drive a demodulator such as a frequency modulation (FM) discriminator.
It is often the case that the second I.F. stages 115 are utilized to obtain large quantities of gain at the second I.F. frequency F.sub.IF2. It is not atypical for second I.F. stages 115 to include amplifiers having gains in excess of 120 db. Since the second I.F. frequency F.sub.IF2 is the lowest intermediate frequency in a dual conversion receiver system it is most economical and advantageous to utilize the second I.F. stages 115 to obtain the majority of the system gain and selectivity.
As stated earlier, the receiver self-quieting phenomenom is the result of harmonics of the first oscillator frequency F.sub.LO1 mixing in any non-linear stage of the receiver with harmonics of the second oscillator frequency F.sub.LO2 in a manner which produces either the first I.F. frequency F.sub.IF1 or more frequently the second I.F. frequency F.sub.IF2. When the first and second local oscillator frequencies F.sub.LO1 and F.sub.LO2 respectively are inadvertently selected to satisfy this condition the result is frequently the presence of a signal in the radio which causes the receiver to respond as though it is receiving an incoming message. In an FM system this can result in the receiver "capturing" itself while ignoring an incoming signal. This condition is known as receiver self-quieting. It is important to note, however, that this phenomenom occurs totally independent of any input signal at frequency F.sub.RF. In other words, if the equation (J.times.F.sub.LO1).+-.(K.times.F.sub.LO2)=.+-.F.sub.IF1 or .+-.F.sub.IF2, where J and K are positive integers, the receiver will respond as though it is receiving an incoming radio frequency signal. One skilled in the art will readily appreciate that even a very small signal level traveling along supply lines, ground lines or signal paths when amplified in the second I.F. stages can severely interfere with proper receiver operation.
By way of an example of this phenomenom, assume that the receiver of FIG. 1 is designed to respond to a signal of 154.585 MHz, has a first I.F. frequency of F.sub.IF1 =10.700 MHz, a second I.F. frequency of 455 KHz, and a first local oscillator frequency F.sub.LO1 of 143.885 MHz. For such a receiver to function properly, two possible second local oscillator frequencies F.sub.LO2 may be utilized: 10.245 MHz (low-side injection) or 11.155 MHz (high side injection). If 10.245 MHz is selected a self-quieting spur occurs as a result of the first harmonic of the first local oscillator frequency mixing with the 14th harmonic of the second local oscillator frequency. In this example, (1.times.F.sub.LO1)-(14.times.F.sub.LO2)=455 KHz. Even though the 14th harmonic of the second local oscillator frequency would presummably be a very small signal, this combination would still be likely to cause severe receiver problems due to the high gain of the second I.F. stages. This is especially true for portable (handheld) receivers or transceivers due to packaging considerations, since in portable equipment, size and weight considerations severely limit the amount of shielding and bypassing which can be implemented to help combat such problems.
In the above example, the low-side injection frequency caused a self-quieting condition to be present. If the high-side injection frequency (11.155 MHz) is utilized no such self-quieting phenomenom occurs. Therefore, one solution to this problem would be to change the second oscillator crystal to 11.155 MHz and re-adjust the second oscillator accordingly to obtain high-side injection. However, it is not always the case that high-side injection is free of self-quieting problems. Many other frequency combinations can be generated which cause self-quieting in a high-side injection receiver system.
To extend the above example somewhat, assume it is also desirable to receive 156.170 MHz on the same receiver. For this particular frequency the first local oscillator frequency is switched to 145.470 MHz. Note however that the first harmonic of 145.470 MHz can mix with the 13th harmonic of 11.155 MHz to obtain the second I.F. frequency of 455 KHz. That is, (1.times.F.sub.LO1)-(13.times.F.sub.LO2)=F.sub.IF2. This set of receive frequencies cannot be properly received on this particular receiver without extreme modifications such as modifying the first or second I.F. frequencies. It is clear, therefore, that there are sets of receiver frequencies which are mutually incompatible in a conventional dual conversion receiver system such as that shown in FIG. 1. It is apparent that the potential user could find himself in the situation of desiring a receiver which may be utilized to receive two channels which his receiver cannot properly process because of self-quieting.
The second problem addressed by this invention is that of receiver frequency stability. For the system of FIG. 1 the first and second oscillators' drift with ambient temperature changes etc., results in receiver performance degradation at the temperature extremes. This is particularly true if the receiver is designed to function over a wide band of input frequencies since this condition necessitates a higher than conventional first intermediate frequency F.sub.IF1. These conditions in turn necessitate a higher than conventional second local oscillator frequency F.sub.LO2. As the second local oscillator frequency F.sub.LO2 rises, its contribution to the overall frequency stability of the receiver increases. This necessitates more complex and expensive second local oscillator designs possibly utilizing extremely high stability and costly crystals.